EDLCs

Electric Double Layer Capacitors (EDLCs) are electric storage devices and serve a similar function as batteries. EDLCs are sometimes called "ultracapacitors" or "supercapacitors."

EDLCs consist of two electrodes (a thin layer of carbon deposited on metal foil current collectors) electrically isolated from each other by a thin separator. The electrodes and separator are immersed in a liquid electrolyte.

EDLCs store energy by grouping positively charged ions in the electrolyte with electrons in the negative electrode (and negative ions with electron holes in the positive electrode). This allows much faster charging and discharging than batteries, which store energy through electrochemical reactions. The energy density in EDLCs has been greatly increased since their early development due to the high surface achieved with activated carbon electrodes; up to half an acre of surface area per gram of carbon (about the weight of a paper clip).

Many research groups are working to bring the energy density of EDLCs up to that of batteries, but they are already superior to batteries in many regards, as shown in the table. In addition to their high power density, they can recharge quickly, they have a significantly longer cycle lifetime, they can withstand harsher environments, and they do not contain toxic components.

The energy consumption of the world is projected to double in the next fifty years. The low carbon foot print of renewables such as wind, solar etc. makes them especially attractive in this era of environmental consciousness. The intermittent nature of energy from renewables is not suitable for commercial and residential grid applications, unless the power can be delivered 24/7, with minimum fluctuation. Hence the viability of renewables as a source of energy critically depends on energy storage technologies such as batteries and ultracapacitors. The energy storage technology is available, nevertheless, the future of renewables as source of energy depends on augmentation of energy and power density, and increasing the long term stability of the storage technologies. Ultracapacitors are unique in that both the energy and power density increases as a square of voltage.

In the E2TAC laboratory, we have been developing novel ionic liquid (IL) electrolytes with wide voltage window and testing them for ultracapacitor applications. Superior properties of ionic liquids such as extremely low vapor pressure, excellent thermal stability, a broad liquid temperature range, and high decomposition potential makes them a preferred choice as an electrolyte for batteries and ultracapacitors. Absence of measurable vapor pressure makes IL a green technology, as they cannot emit volatile organic compounds.

Shown below (1) is the parent compound, 1,1'-Spirobipyrrolidinium (SBP) salt, which exhibits high conductivity and solubility, and has been reported for ultracapacitor application. Various modifications (2) such as introduction of oxygen, and alkyl groups (R2 - R5) to the parent compound were incorporated to generate novel ionic liquids with lower melting point and viscosity, while preserving the conductivity.

a)Type-1. Rest of the salts are of Type-2

Salt

X

R2

R4

R5

A-

Reaction mechanism

aSBP BF4

-

-

-

-

BF4

SBP Cl + HBF4 → SBP BF4

OP BF4

CH2

H

H

H

BF4

OP Cl + HBF4 → OP BF4

OP BF3C2F5

CH2

H

H

H

BF3C2F5

OP BF4 + KBF3C2F5 → OP BF3C2F5

OP NTf2

CH2

H

H

H

NTf2

OP Cl + LiNTf2 → OP NTf2

2MOP BF4

CH2

Me

H

H

BF4

2MOP Cl + HBF4 → 2MOP BF4

2MOP BF3C2F5

CH2

Me

H

H

BF3C2F5

2MOP BF4 + KBF3C2F5 → 2MOP BF3C2F5

2MOP NTf2

CH2

Me

H

H

NTf2

2MOP BF4 + LiNTf2 → 2MOP NTf2

2EOP BF4

CH2

Et

H

H

BF4

2EOP Cl + HBF4 → 2EOP BF4

4MOP BF4

CH2

H

Me

H

BF4

4MOP Cl + HBF4 → 4MOP BF4

5MOP BF4

CH2

H

H

Me

BF4

5MOP Cl + HBF4 → 5MOP BF4

5MOP NTf2

CH2

H

H

Me

NTf2

5MOP BF4 + LiNTf2 → 5MOP NTf2

SBO BF4

O

H

H

H

BF4

SBO Cl + HBF4 → SBO BF4

A variety of novel oxygen and alkyl group containing spirobipyrrolidinium compounds were investigated for room temperature ionic liquid applications. Six novel cations, namely, OP, 2EOP, 2MOP, 4MOP 5MOPand SBO, were synthesized by employing aldehydes and haloalcohols. All together, eleven novel salts were produced. Threetypes of anions, BF4, BF3C2F5 and NTf2 were synthesized. The BF4 and NTf2 analogs were produced by conversion of chlorides, while BF3C2F5 analog was generated by metathesis of BF4.

Melting point and voltage window were determined from TGA and linear sweep voltammetry. The melting point reduced from 189oC (SBP BF4) to below 100oC in six salts, and was below 30oC (Room Temperature Ionic Liquid) in four salts. Potential window measurements was performed at 0.65M solution dissolved in AN for each salt. The parent compound SBP BF4 had a voltage window of 7.3 V. Six out of eleven salts had a potential window ³ 7.0 V. The potential window varied from 5.9 to 7.3 V. The potential window of commercial workhorse 0.65 M TEABF4 in AN was c.a. 5.5 V. Conductivity measurement was performed at various concentrations (0.65M to 5.0M in AN), and a conductivity maximum was observed around 2M concentration for all. The conductivity (at 2M) of all the salts was in the range ~48-55 mS / cm, as compared with 65 mS / cm for SBP BF4. The thermal stability of various salts was found to reduce from c.a. 500oC to 300-420oC range, which is more than adequate for most of the high temperature ultracapacitor applications. [This work has been accepted for publication in Helvetica Chimica Acta 2009]

The performance of oxazolidine-3-spiro-1'-pyrrolidinium tetrafluoroborate (OPBF4) was tested in an ultracapacitor cell, and characteristics including potential window, conductivity, capacitance, and concentration were compared with commercially available TEABF4. OPBF4 (1-3M) had higher voltage window ~ 5V, as compared to TEABF4 (1-1.5M) which was ~4.5-4.6 V. The compact nature of OP cation led to 10% higher specific capacitance in OPBF4 as compared with TEABF4 at 3V. However, this was also responsible for lower electrode breakdown potential in OPBF4 due to the reaction between OPBF4 and functionalities in activated carbon electrode. This is the first report of testing OPBF4 compound in an EDLC device, and it qualifies as a reasonable alternative to TEABF4 for high performance ultracapacitors. [This work is published in Electrochemistry Communications: Vol. 11, Issue. 3, Page: 680-683, 2009.]